terahertz communication for satellite networks
TRANSCRIPT
TERAHERTZ COMMUNICATION FOR SATELLITE NETWORKS
by
Pulkit Hanswal
June 1, 2018
A thesis submitted to the
Faculty of the Graduate School of the University at Bufalo, State University of New York
in partial fulfllment of the requirements for the
degree of
Master of Science
Department of Electrical Engineering
Acknowledgements
I would like to express my sincere gratitude to my advisor Dr. Josep M. Jornet
for the continuous support during my Master thesis and related research, for his
patience, motivation and immense knowledge. The door to Dr. Josep M. Jornet
ofce was always open whenever i ran into a trouble spot or had a question about
my research or writing. His guidance helped me in all the time of research and
writing of this thesis. I could not have imagined having a better advisor and
mentor for my Master thesis.
Besides my advisor, I would like to thank Dr. Elena Bernal Mor for her
encouragement, advise throughout my years of study and recommending me for
the Master thesis to Dr. Josep M. Jornet.
Finally, I must express my profound gratitude to my parents for providing
me with unfailing support and continuous encouragement. This accomplishment
would not have been possible without them.
iii
Contents
Acknowledgements iii
List of Figures viii
List of Tables ix
Abstract x
1 Introduction 1
1.1 Basic Characteristics of Satellites . . . . . . . . . . . . . . . . . . . 1
1.1.1 System Elements . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 Types of Satellite Services . . . . . . . . . . . . . . . . . . . 4
1.1.3 Benefts of Satellite Communication . . . . . . . . . . . . . 7
1.1.4 Frequency Spectrum Allocations . . . . . . . . . . . . . . . 8
1.2 Satellite Classifcation . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.1 GEO Satellites . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.2.2 LEO Satellites . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.3 MEO Satellites . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.3 Inter-Satellite Communication . . . . . . . . . . . . . . . . . . . . . 14
1.3.1 ISL Frequency Bands . . . . . . . . . . . . . . . . . . . . . 15
1.4 Terahertz Based ISL . . . . . . . . . . . . . . . . . . . . . . . . . . 18
iv
Contents Contents
1.4.1 Terahertz Communication . . . . . . . . . . . . . . . . . . . 18
1.4.2 Summary of Contributions . . . . . . . . . . . . . . . . . . 19
2 Terahertz Wave Propagation in ISL 20
2.1 Spreading Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 Absorption Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3 Scattering Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.4 Fading Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3 Noise in Terahertz based ISL 30
3.1 Thermal Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Transceiver Noise Sources . . . . . . . . . . . . . . . . . . . . . . . 31
3.2.1 Amplifer Noise Temperature . . . . . . . . . . . . . . . . . 31
3.2.2 Amplifers in Cascade . . . . . . . . . . . . . . . . . . . . . 33
3.2.3 Noise Temperature of Absorptive Networks . . . . . . . . . 34
3.3 Antenna Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3.1 Brightness Temperature . . . . . . . . . . . . . . . . . . . . 36
3.3.2 Cosmic Microwave Background Radiation . . . . . . . . . . 37
3.3.3 Thermal Radiation . . . . . . . . . . . . . . . . . . . . . . . 38
3.3.4 Sky Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.4 Total System Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.5 Signal-to-Noise ratio . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4 Link Budget Analysis 43
4.1 Antenna Gain Required - Lower Bound . . . . . . . . . . . . . . . 44
4.2 Antenna Gain Required - Upper Bound . . . . . . . . . . . . . . . 48
4.3 Terahertz ISL vs Optical ISL . . . . . . . . . . . . . . . . . . . . . 50
5 Additional Challenges in THz based ISL 53
5.1 Space Debris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
v
Contents Contents
5.1.1 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5.1.2 Simulation Model . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.3 Power Delay profle . . . . . . . . . . . . . . . . . . . . . . . 58
5.2 Doppler Spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6 Conclusions and Future Work 62
Bibliography 63
vi
List of Figures
1.1 Elements of the space segment [1] . . . . . . . . . . . . . . . . . . . 2
1.2 Ground segment [1] . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Satellite Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Fixed satellite service [2] . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Broadcasting satellite service [2] . . . . . . . . . . . . . . . . . . . 5
1.6 Mobile satellite service [28] . . . . . . . . . . . . . . . . . . . . . . 6
1.7 GPS constellation [29] . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.8 Inclination classifcation [30] . . . . . . . . . . . . . . . . . . . . . . 9
1.9 LEO,MEO and GEO.Only one orbit per altitude is illustrated,
even though LEO and MEO satellites require multiple orbits for
continuous service [2] . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.10 GEO satellite system [2] . . . . . . . . . . . . . . . . . . . . . . . . 11
1.11 Iridium constellation [31] . . . . . . . . . . . . . . . . . . . . . . . 13
1.12 TDRS system [32] . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.13 RF based ISL [35] . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.14 Optical communication based ISL [33] . . . . . . . . . . . . . . . . 18
2.1 THz wave propagation from an elemental antenna [1] . . . . . . . . 21
2.2 Diagram of atmospheric windows [34] . . . . . . . . . . . . . . . . 23
2.3 Absorption loss at THz frequency due to water vapor in dB/km . . 24
vii
List of Figures List of Figures
2.4 Layers of atmosphere [11] . . . . . . . . . . . . . . . . . . . . . . . 26
3.1 Non ideal resistor & its equivalent scheme [1] . . . . . . . . . . . . 31
3.2 Non ideal LNA & its equivalent scheme [1] . . . . . . . . . . . . . . 32
3.3 LNA in cascade [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.4 Large bright noise source with brightness temperature �� [2] . . . 36
3.5 Small bright noise source [2] . . . . . . . . . . . . . . . . . . . . . . 37
3.6 Antenna temperature of an ideal ground based antenna. The an-
tenna is assumed to have very low beamwidth without losses and
sidelobes [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.1 Gain required for SNR of 10 dB vs Distance (Lower Bound) . . . . 45
4.2 Beamwidth vs Distance (Lower Bound) . . . . . . . . . . . . . . . 46
4.3 Beam diameter vs Distance (Lower Bound) . . . . . . . . . . . . . 47
4.4 Gain required for SNR of 10 dB vs Distance (Upper Bound) . . . . 48
4.5 Beamwidth vs Distance (Upper Bound) . . . . . . . . . . . . . . . 49
4.6 Beam diameter vs Distance (Upper Bound) . . . . . . . . . . . . . 49
4.7 Beam diameter in Optical ISL . . . . . . . . . . . . . . . . . . . . . 51
4.8 Optical ISL vs THz ISL . . . . . . . . . . . . . . . . . . . . . . . . 52
5.1 Geometry of the Network considered. Refection occuring at point A. 55
5.2 Space debris enclosed in Transmit & Receive Beam,� = 10-9 per �2 56
5.3 Multipath components as function of path delay, � = 10-9 per �2 . 57
5.4 Power delay profle vs time . . . . . . . . . . . . . . . . . . . . . . 59
viii
List of Tables
1.1 Frequency band allocations . . . . . . . . . . . . . . . . . . . . . . 9
1.2 Frequency bands for RF based ISL . . . . . . . . . . . . . . . . . . 17
4.1 Parameters - Lower bound . . . . . . . . . . . . . . . . . . . . . . . 44
4.2 Parameters - Upper bound . . . . . . . . . . . . . . . . . . . . . . . 48
ix
Abstract
Satellite communication, no longer a marvel of human space activity, has become
a fact of everyday life. Satellites are used extensively for variety of communication
applications. In recent years, there is growing interest in the feld of satellite
networks which enables a whole new class of missions for navigation, communication
and remote sensing. Consequently, there is need for innovative methodologies to
meet the increasing demand for high speed wireless communication. Terahertz
band communication is envisioned to be the key technology to satisfy the need
for high data rates. Due to the large availability of bandwidth at terahertz band,
wireless terabit per second links are expected to become reality very soon.
The objective of this work is to study the feasibility of using terahertz band for
inter-satellite communication. We have developed a propagation model considering
several impairments which can afect the propagation of terahertz wave in inter-
satellite links. We have also analyzed noise sources which can impact the terahertz
based communication and calculated the efective noise temperature at the satellite
antenna. A link budget analysis to calculate the gain of the antenna required
to maintain the quality of the link is conducted and a comparative analysis
of terahertz based and optical based inter-satellite communication on the basis
of pointing requirements is also done. Lastly, we have addressed some of the
additional challenges that we need to overcome for implementing terahertz based
inter-satellite communication.
x
CHAPTER 1
Introduction
A satellite is a body that moves around another body (usually much larger) in
a mathematically predictable path called an orbit. Satellites orbiting around
earth can revolve either in circular or elliptical path. Satellites are used for many
purposes. Common types include Earth observation satellites, navigation satellites,
communication satellites, weather satellites, and space telescopes.We mainly focus
in this text on communication satellites.
1.1 Basic Characteristics of Satellites
A communication satellite is a repeater satellite orbiting around earth which
enables multiple users with appropriate earth stations to deliver or exchange
information. Two earth stations which are very distant geographically can make
use of communication satellite which can relay and amplify the signals via a
transponder. The source earth station sends a transmission to the satellite. This
is called up-link communication. The satellite transponder then amplifes or relays
the signal and send it down to the destination earth station. This is called the
1
Tracking. telemetry
and command (TT&C) station
"'~
Deployment
,,..__-i-__ ..... ,
'Transfer l orbit
phase
Chapter 1. Introduction 1.1. Basic Characteristics of Satellites
down-link communication.
Figure 1.1: Elements of the space segment [1]
1.1.1 System Elements
A communication satellite system can be divided into two major parts, the space
segment and the ground segment.
Space Segment
The space segment includes the satellite, the launch station and tracking, telemetry
and command (TT&C) station. Figure 1.1 depicts the main elements of the space
segment. Deployment of a satellite in an orbit is accomplished by the help of a
spacecraft manufacturer and launch agency. After the satellite is placed in the
proper orbit, it is vital to monitor the satellite for the duration of its mission. This
task is accomplished by TT&C station (or stations). The TT&C station establishes
a monitor link to provide essential management and control functions for the safe
2
Chapter 1. Introduction 1.1. Basic Characteristics of Satellites
operation of the satellite. The link between the satellite and TT&C station is
usually separate from the link used for user communication.Tracking, telemetry
and control station is specialized earth terminal facility specifcally designed for
the complex operation required to maintain a satellite in orbit.
Ground Segment
The ground segment of the communication satellite system consists of earth stations
that utilize the communication capabilities of space segment. The earth stations
communicate with satellites to meet the communication needs of the user. It is
important to note that the term earth station is an accepted term that include
the communication stations on the ground, in the air (on airplanes), or on the sea
(on ships). Figure 1.2 depicts a typical ground segment; a single satellite is shown
which acts as a relay to establish the links from one earth station to another.
Figure 1.2: Ground segment [1]
It is important to note that depending upon the coverage area of a satellite,
3
• I I - I I ISL
Chapter 1. Introduction 1.1. Basic Characteristics of Satellites
there can be satellite to satellite link in a satellite system. Figure 1.3 depicts
Inter-satellite links (ISL) and Ground-satellite links (GSL).
Figure 1.3: Satellite Links
1.1.2 Types of Satellite Services
Satellite services are broadly categorized into
∙ Fixed-satellite service (FSS) - This is a type of radio communication service
between earth stations at specifc fxed positions. There are many subdivisions
within FSS, e.g., the FSS provide links for telephone networks as well as for
transmitting TV signals to cable companies. Figure 1.4 depicts a typical
example of fxed satellite service.
4
Chapter 1. Introduction 1.1. Basic Characteristics of Satellites
Figure 1.4: Fixed satellite service [2]
∙ Broadcast-satellite service (BSS) - In this service, the signal transmitted
by the satellite is intended mainly for direct broadcast to the home. It is
sometimes referred to as direct broadcast satellite (DBS) service. One of
the most common example of BSS is Direct to Home(DTH) television.As
shown in Figure 1.5, satellite is broadcasting signal to all the users within
the satellite coverage.
Figure 1.5: Broadcasting satellite service [2]
∙ Mobile-satellite service (MSS) - In this type of service there is communication
between mobile earth stations with the help of one or more satellites. MSS
would include land mobile, maritime mobile, and aeronautical mobile. Figure
5
Chapter 1. Introduction 1.1. Basic Characteristics of Satellites
1.6 shows a mobile satellite phone.
Figure 1.6: Mobile satellite service [28]
∙ Navigation satellite service (NSS) - A navigation satellite service uses a net-
work of satellites (typically 18-30 satellites for global coverage) for providing
geo-spatial positioning. An NSS with global coverage is termed as global
navigation satellite system (GNSS). The most famous example of NSS is
United States’ Global Positioniing System (GPS). Figure 1.7 illustrates a
GPS constellation
6
....... ... ' "
.... .. •••
I I \ • -i r • \ "
,, .. "'· .... • ..... .,..#) -~ . ...... ..
Chapter 1. Introduction 1.1. Basic Characteristics of Satellites
Figure 1.7: GPS constellation [29]
∙ Meteorological satellite service - Lastly, satellites are also used for providing
meteorological services such as weather forecast or services for search and
rescue missions.
1.1.3 Benefts of Satellite Communication
The use of the satellites for communication has become a fact of everyday life.
Satellites are used extensively for a variety of communication applications. It is
evidenced by many homes which are equipped with dishes used for the reception
of satellite television. Satellites also form an essential part of telecommunications
systems worldwide, carrying large amounts of data and voice trafc. As mentioned
in [2], some of the advantages of using satellites are as follows
1) Wide Coverage Area (Country, Continent or Globe) - When designed properly,
a satellite can serve region of any size that can see it. However, in terrestrial
systems, the coverage area is limited usually within a country.
2) Independence from Terrestrial Infrastructure - By providing wide coverage area
and acting as repeater station in space, satellite aids in reducing the terrestrial
7
Chapter 1. Introduction 1.1. Basic Characteristics of Satellites
infrastructure. Two earth station far distant geographically can communicate
without any external connections using satellite (or satellites). It becomes really
attractive in remote areas where the terrestrial infrastructure is poor.
3) Uniform Service Characteristics - Footprint of the satellite defnes the service
area of the satellite. As we will discuss later in this chapter, footprint depends
upon the altitude of the satellite. Within the footprint of the satellite, services are
delivered in precisely the same form. However, in terrestrial systems the quality
of the received signal depend upon the location, e.g., remote areas with poor
terrestrial infrastructure does not have good quality of received signal.
4) Wide Bandwidth Availability- The bandwidth allocated for the satellite services
is generally high. As a result, satellite users enjoy ample amount of bandwidth.
Since, there is a direct relationship between bandwidth and data rate. An increase
in bandwidth can enhance the speed of communication.
5) Rapid Installation & Low Cost - As mentioned in [2], once the satellite system is
operational, the earth station in the ground segment can be installed very quickly.
This is because unlike terrestrial system it does not require extensive planning
such as laying down cables and towers throughout the area. Furthermore, the cost
of constructing a single site is quite modest.
1.1.4 Frequency Spectrum Allocations
Frequency allocation to satellite services is a complicated process which requires
planning and coordination. Frequency band allocation is done by the International
Telecommunication Union (ITU), a specialized agency of the United Nations. Table
1.1 lists the frequency bands assigned by ITU for satellite services.
8
L
s
C
X
Ku
Ka
V
Frequency Range 1.535-1.56 GHz DL; 1.635-1.66 GHz UL
2.5-2.54 GHz DL; 2.65-2.69 GHz UL
3.7-4.2 GHz DL; 5.9-6-4 GHz UL
7.25-7.75 GHz DL; 7.9-8.4 GHz UL
10-13 GHz DL; 14-17GHzUL
18-20 GHz DL; 27-31GHzUL
6oGHzDL; 50GHzUL
General Application
MSS,NSS
MSS, Deep space research
FSS
FSS, Earth observation, Meteorological satellite service
FSS,BSS
FSS,BSS
Open band
Orbit
-@ '~~ (3) a. Equatorial-orbit satellite b. Inc lined-orbit satellite c. Polar-orbit satellite
Chapter 1. Introduction 1.2. Satellite Classifcation
Table 1.1: Frequency band allocations
1.2 Satellite Classifcation
There are many ways to classify satellites. Some of them include inclination
classifcation, synchronous classifcation, altitude classifcation.
a) Inclination classifcation - This divides satellites on the basis of inclination with
reference to the equatorial earth plane. Figure 1.8 illustrates the types of satellites
based on inclination classifcation.
Figure 1.8: Inclination classifcation [30]
b) Synchronous classifcation - In this type of classifcation, satellites are divided
on the basis of the synchronization between the orbital period and rotational period
of the earth. A geosynchronous orbit (GSO) satellite has an orbital period equal
to the rotational period of the earth. Geosynchronous orbit (GSO) satellites rotate
9
Chapter 1. Introduction 1.2. Satellite Classifcation
around the earth at an altitude of 35,786 km. The synchronization of the rotational
and orbital period means that the satellite return to the same position in the sky
after a period of one day. The path of the satellite over the course of one day
depends upon the inclination and eccentricity of the GSO.
c) Altitude classifcation - This divides satellites by their altitude from the earth.
Figure 1.9 provides three classical Earth-orbit schemes. The commonly used
altitude classifcations are Geostationary earth orbit (GEO) satellites, Medium
earth orbit (MEO) satellites and Low earth orbit (LEO) satellites. We mainly
focus in this text on the classifcation based on altitude.
Figure 1.9: LEO,MEO and GEO.Only one orbit per altitude is illustrated, even though LEO and MEO satellites require multiple orbits for continuous service [2]
1.2.1 GEO Satellites
Geostationary earth orbit is a special case of GSO, which is circular and inclined
at 0∘ to Earth’s equatorial plane. Satellites in GEO revolve at an altitude of about
35,786 km. A satellite in a GEO appears always at the the same point in sky to the
observers on the surface. They are mainly used for FSS, BSS and meteorological
satellite service.
10
I/_,,,--------------------------------+
' ---
•.. ~·. ···• ... -,_______ · , __ '-4' )
---------- ·,.:-_ ,// -------------- ________ ,.,..,.
Chapter 1. Introduction 1.2. Satellite Classifcation
Figure 1.10: GEO satellite system [2]
Advantages
1. Due to very high altitude, GEO satellites have very large coverage area. It
requires only 3 satellites to cover every part of earth. Figure 1.10 shows a
system of three geostationary communication satellites.
2. As GEO satellites have 24 hour view of a particular location, they are highly
desirable for services which require fxed antenna positions, e.g., DTH, TV
and radio broadcasting
3. As the coverage area of GEO satellites is very wide, there is no need of
handing over the signal from one satellite to another.
4. The Doppler spread in communication between satellite and earth station is
minimal as there is no relative motion between them.
Disadvantages
1. Due to large distance from earth, there is a very high latency which makes
it unft for voice and data communications.
2. Due to large footprints we cannot reuse frequency.
3. As we will see in the later chapters, more distance leads to high propagation
loss which eventually leads to poor quality of signal at the receiver.
4. In order to maintain the quality of the link, high transmit power is needed
11
Chapter 1. Introduction 1.2. Satellite Classifcation
which is a problem for battery powered devices.
1.2.2 LEO Satellites
LEO satellites revolves in geocentric (around earth) orbits ranging in altitude from
180 km – 2000 km (1,200 mi). LEO satellites do not stay in fxed position relative
to the surface, and are only visible for 15 to 20 minutes each pass. They have
rotation period of 90 to 120 minutes, with speed 20000 to 25000 km/h. They are
mainly used for FSS and MSS.
Advantages
1. Due to it’s proximity to earth compared to GEO satellite, there is better
signal strength in comparison to GEO satellites.
2. They need lesser transmit power for communication in comparison to GEO
satellites.
3. There is very low latency of around 10 milliseconds.
4. They have smaller footprints so there can be frequency reuse.
Disadvantages
1. A network of LEO satellites (50-200),which is also known as satellite con-
stellation, is required to cover entire earth because of the small footprint.
There are satellite to satellite links in the case of LEO satellite network for
handing over the information.
2. The lifetime of LEO satellite is shorter than GEO satellites (5-8 years). This
is because of the atmospheric drag which causes gradual orbital deterioration.
3. There is Doppler spread in the communication between LEO satellite and
earth station because of the relative motion between them.
4. There is need for routing from satellite to satellite to base stations for global
12
Chapter 1. Introduction 1.2. Satellite Classifcation
coverage.
5. There is requirement for a mechanism for correct handover of the signal from
one satellite to another.
Figure 1.11: Iridium constellation [31]
Some of the examples of LEO satellite systems are as follows
∙ Iridium system - It has 66 satellite in 6 LEO orbits which is used to provide
direct worldwide communication, i.e., voice, data paging, fax. Figure 1.11
illustrates an iridium satellite constellation.
∙ Teledesic - It provides fber-optic like (broadband channels, low error rate,
and low delay) communication.It has 288 satellites in 12 LEO polar orbits.
1.2.3 MEO Satellites
MEO satellites revolves in geocentric orbits ranging in altitude from 2000 km –
35,786 km. MEO satellites are visible for much longer periods of time than LEO
satellites, usually between 2 – 8 hours. They are used mainly for NSS.
13
Chapter 1. Introduction 1.3. Inter-Satellite Communication
Advantages
1. A MEO satellites’ wider footprint and slower movement in comparison to
LEO satellite means fewer satellites are needed in MEO network than in
LEO network. MEO satellite networks also make use of satellite to satellite
links.
2. Shorter time delay and stronger signal than GEO satellite.
3. It requires fewer handover than LEO network.
Disadvantages
1. Delay is about 70-80 milliseconds.
2. Needs higher transmit power than LEO satellites.
A well known example of MEO satellite network is GPS.
∙ GPS comprises of 24 satellites in six orbits. They are used for land, sea
and air navigation to provide time and locations for vehicles and ships. The
satellites are placed in orbit in such a way that minimum four satellites are
visible from any point on the earth.
1.3 Inter-Satellite Communication
Inter-satellite communication is mainly used for providing global coverage with the
help of satellite to satellite links. There are two types of ISL, intra-orbital links
and inter-orbital links. Intra-orbital links connects the satellites on the same orbit
while inter-orbital links connects the satellites on diferent orbits. Inter-satellite
communication becomes a necessity in the case of LEO and MEO satellite networks
because of their smaller footprint and the need for global coverage.Apart from LEO
and MEO satellite networks, they can also be used for communication between
GEO satellite and LEO satellite. An example of such a satellite system is Tracking
14
Chapter 1. Introduction 1.3. Inter-Satellite Communication
and Data Relay Satellite system (TDRS). There are 9 Tracking and Data Relay
satellites which sit at about 35,400 km (almost GEO) above the earth.Occasionally,
satellites present in the LEO orbit are unable to pass along their information to
Earth in the case of unclear view of the ground station. In such a case, TDRS
serves as a way to pass along the satellite information. Figure 1.12 illustrates a
TDRS system. Some of the other well known examples of satellite constellations
which make use of ISL are iridium and teledesic. Apart from providing global
coverage, ISL helps considerably in reducing the terrestrial infrastructure. This
is because we need only 1 up-link and 1 down-link per direction for any point to
point communication.
Figure 1.12: TDRS system [32]
1.3.1 ISL Frequency Bands
Currently, there are two types of ISL based upon the technology used for com-
munication between satellites. Radio Frequency (RF) based ISL make use of
microwaves (0.3 GHz-30 GHz) and millimeter waves (30 GHz - 300 GHz) while
15
Ka-band ---~ ---~ ·band --- ------ Iridium Satellites ---...... ', l-band
' ' ' ' -~ ~autical
, I
I I
/ Ka-band
I I I
Chapter 1. Introduction 1.3. Inter-Satellite Communication
optical communication based ISL make use of visible spectrum, i.e., wavelength
range of about 390 nm to 700 nm, for communication.
Radio Frequency based ISL
Long term experience with radio equipment developed for space to ground links
makes RF based ISL more easier to implement in space. As described in [7], in
comparison to optical based ISL, RF based ISL appears more reliable. However, due
to less availability of bandwidth the data rate achievable in RF link is lower than
the optical link.As we will see in the later chapters, due to low carrier frequency
RF links can provide omni directional coverage and also has higher beam width
than optical links which reduces the need for highly accurate acquisiton, tracking
and pointing (ATP) subsystem. Although, this high beam width makes RF links
subjected to interference and multi path propagation. Figure 1.13 illustrates an
iridium satellite system which makes use of RF based ISL.
Figure 1.13: RF based ISL [35]
Table 1.2 indicates the frequency bands allocated to RF based ISL by radio
communication regulations. As we will see in chapter 2, these allocations have
been done because of the strong molecular absorption by the atmosphere at these
16
Chapter 1. Introduction 1.3. Inter-Satellite Communication
frequencies. Using these frequencies for ISL provide protection against interference
between terrestrial system and satellite network.
Band Frequency Range 22.55 GHz - 23.55 GHz
Ka 24.45 GHz - 24.75 GHz 32 GHz - 33 GHz 59 GHz - 64 GHz V 65 GHz - 71 GHz
Table 1.2: Frequency bands for RF based ISL
Optical Communication based ISL
Optical communication based ISL has the advantage of providing high speed
communication with the data rate of the order of gigabits per second (Gbps).
However, due to high carrier frequency they have very low beam width. As
mentioned in [10], the challenging part in using optical communication for ISL
is the requirement of an highly accurate ATP subsystem for establishing and
maintaining the communication link. In order to maintain a link in optical
communication based ISL, a direct line of sight is required between transmitter
and receiver. The process of maintaining the link becomes even more complex due
to the rapid relative motion between two satellites. Although, the low beam width
of the optical based ISL aids in receiving interference free signal and also reduces
the possibility of interception. They can also be used for communication between
spacecrafts when they reach very short distances with respect to each other which
guarantees very high position accuracy. A well known example of the optical based
ISL is the TDRS system which makes use of laser for communication.
17
Chapter 1. Introduction 1.4. Terahertz Based ISL
Figure 1.14: Optical communication based ISL [33]
1.4 Terahertz Based ISL
1.4.1 Terahertz Communication
Terahertz band (0.1 THz-10 THz) communication is considered to be the key
technology to support the increasing demand for high speed wireless communication
in the recent years. Due to the large availability of bandwidth at THz band, wireless
terabit per second (Tbps) links are expected to become reality very soon. As we will
see in the next chapter, this very large bandwidth comes at the cost of a very high
propagation loss. The high propagation loss is primarily due to the high molecular
absorption at THz frequencies and also due to the increase in spreading loss at
higher frequency. The advancements in the feld of communication can compensate
for the propagation loss at higher frequencies but the high molecular absorption
remains an issue. Consequently, for the time being, THz band communication
in terrestrial environment is mainly constrained to very short (e.g., on chip) and
short (e.g., in a room) links.
18
Chapter 1. Introduction 1.4. Terahertz Based ISL
In this work, we propose satellite to satellite links using THz frequencies. Due
to the high availability of bandwidth, THz based ISL can enable higher data
rate in comparison to RF based ISL without having to switch to a diferent set
of hardware such as lasers for optical communication. Also, due to the high
frequency of the communication system, the size of the antenna required for THz
based ISL is very less which is highly favorable to the smaller satellite systems.
Terahertz spectrum also provides inherent isolation from terrestrial RF systems
and thus eliminates potential radio frequency interference from ground systems.
Furthermore, as explained in [7], because of the presence of multiple GHz channel
bandwidth terahertz wireless system has very low complexity system design with
simplest modulation schemes in comparison to traditional radio architectures.As
we will show in the future chapters, THz based inter-satellite communication also
have higher efciency than optical communication based ISL.
1.4.2 Summary of Contributions
In the following chapters, we will develop a propagation model and learn about
the factors afecting the THz wave propagation in ISL. In Chapter 3, we will look
into the noise sources which can afect THz based inter-satellite communication.
In Chapter 4, we will conduct a link budget analysis and do a comparative study
between optical and terahertz based ISL. Finally, we will address some of the major
challenges that we need to overcome for implementing THz based inter-satellite
communication.
19
CHAPTER 2
Terahertz Wave Propagation in ISL
When a THz wave propagates through a medium there can be several impairments.
Some of the important impairments which afect THz wave propagation will be
described in this chapter.
2.1 Spreading Loss
Spreading loss indicates the amount of power that is lost when an EM wave
propagates in an environment. Once THz wave is several dimensions away from
its emitter, its energy propagates out radially. Figure 2.1 depicts THz wave
propagation from an elemental antenna.
20
Maximum propagation
Antenna reference
axis ' ' ' : '
I I I I I I I I I I I
Maximum propagation
Chapter 2. Terahertz Wave Propagation in ISL 2.1. Spreading Loss
Figure 2.1: THz wave propagation from an elemental antenna [1]
Consider an isotropic point source with a transmitting power of ��. At a
distance d from the source, the radiated power is uniformly distributed over the
surface area of a sphere.
We can write the received power �� at distance d is given by
1 �� = ���� (2.1)4��2
��,
where �� is gain of the transmitter antenna and �� is the efective aperture of the
receiver antenna. We can also write the gain of the receiver antenna �� by
�� =4 �
� 2
��, (2.2)
where � is the wavelength of the THz wave.
After substitution we get,
( )2�
�� = ������ , (2.3)4��
which we can further rewrite to
( )2�
�� = ������ , (2.4)4����
21
Chapter 2. Terahertz Wave Propagation in ISL 2.2. Absorption Loss
where c is the speed at which THz wave propagates in a medium & �� is the carrier
frequency of the wave. The spreading loss is given by
( )21 4���� � = . (2.5)
���� �
We can further express this equation in decibels(dB).
���� = ���� + ���� + ���� − ��� . (2.6)
The propagation model described by equation (2.6) is known as Friis prop-
agation model. From equation (2.5) we can see that as we increase the carrier
frequency the power received at the receiver antenna decreases. This equation
explains the cause of high spreading loss, when communicating at THz frequencies.
We can also see that this spreading loss can be compensated by increasing the
gain of the receiver and transmitter antennas. The propagation loss also increases
as we increase the distance between transmitter and receiver. The propagation
model described above is also known as free-space model. This model very well
fts our scenario. We will be using this model to conduct link-budget analysis in
the Chapter 4.
2.2 Absorption Loss
Atmospheric absorption of electromagnetic radiation happens when the energy of
the photon present in the radiation is taken by the molecules (typically electrons of
atom) in the atmosphere. This electromagnetic energy absorbed by the molecules
is usually converted into another form such as thermal energy. Figure 2.2 illustrates
the wavelengths at which the EM waves are note able to penetrate the Earth’s
atmosphere. The primary gases which leads to atmospheric absorption of energy
are water vapor, ozone and carbon dioxide. As shown in Figure 2.2, the wavelengths
22
-~
:1 ,]MC:i f {f .. 0 <> EO <
I I I I I I 01 nm 1"m 10nm 100nm
Gamma Raya, X•Raya and Ultrav ic»et Llgh1 blocked by the upl)ef • 1mOSl)hefe (b .. 1 obHrvttd from apace).
1 ,m 101,1m
Viaibk Ligh1 observable f rom Earth, wi1h some •tmospheric distortion.
1001,1m 1 mm 'cm Wavelength
Mos1ot the lnfrarttd ap.tetrum • bsorbed by abnospheric gasses (bes1 -~ ... from space).
10cm 1m 10 m
Radio W.Vff observable from Earth.
I I 100m 1 km
Long-wavelength R.ldio Wav .. blocked.
Chapter 2. Terahertz Wave Propagation in ISL 2.2. Absorption Loss
in the range of 30 �m to 3000 �m (Terahertz range) are completely absorbed by
Earth’s atmosphere.This absorption is mainly due to the water vapor present in
the atmosphere.
Figure 2.2: Diagram of atmospheric windows [34]
Figure 2.3 shows attenuation due to water vapor in the range of 0.1 THz
to 10 THz. It shows that the attenuation can go as high as 1000 dB for some
specifc frequencies. Due to this very strong absorption we are unable to use THz
frequencies for terrestrial systems.
23
1800
1600
I 1400 ii3 ~ 1200 <JJ
gi ..J 1000 C 0
li 800
~ .n <x: 600
400
200
2 3 4 5 6 8 9 10
Frequency ITHz]
Chapter 2. Terahertz Wave Propagation in ISL 2.2. Absorption Loss
Figure 2.3: Absorption loss at THz frequency due to water vapor in dB/km
Layers of Earth’s Atmosphere
In order to analyze the impact of atmospheric absorption on THz based ISL, we
should frst study about the layers of atmosphere. Earth’s atmosphere consists
of series of layers each of which has its own properties. Moving upward from
the ground level, these layer are named as troposphere, stratosphere, mesosphere,
thermosphere and exosphere. Figure 2.4 depicts all the layers of atmosphere.
∙ Troposphere - The troposphere is the lowest layer of the atmosphere. It
extends upward from ground level to about 10 km (33,000 feet) above sea
level. 99% of the water vapor in the atmosphere is found in the troposphere.
As we move up in the troposphere temperature gets colder.
∙ Stratosphere - The stratosphere extends from the top of troposphere to about
50 km above the ground. This layer contains the ozone molecules which
absorb the high-energy ultraviolet (UV) light from the Sun, which is then
converted into thermal energy. The temperature of the stratosphere gets
24
Chapter 2. Terahertz Wave Propagation in ISL 2.2. Absorption Loss
warmer as we move upwards.
∙ Mesosphere - The next layer up is called mesosphere. It extends upward to
a height of about 85 km above earth. Unlike the stratosphere, temperature
once again grows colder as we rise up through the mesosphere. The top of
the mesosphere is the coldest place in earth’s atmosphere with a temperature
around -85∘C (188∘K). The air density in mesosphere is far too low to breathe.
Also, the air pressure at the bottom of the mesosphere is 1% of the pressure
at sea level.
∙ Thermosphere - Above the mesosphere is the thermosphere. As mentioned
in [11], this layer is completely free of water vapor. High energy X-rays and
UV radiation from the Sun are absorbed in the thermosphere. As a result,
temperature of the thermosphere can range from about 500∘C (773∘K) to
2000∘C (2273∘K). However, as we will see in the next chapter, due to very
low density of the air the notion of the temperature is not meaningful at this
layer. The top of the thermosphere lies between 500 to 1000 km depending
upon the amount of the radiation coming from the sun. LEO and many
MEO satellites orbit at this layer.
∙ Exosphere - The exosphere is the topmost layer of the atmosphere.The air is
very, very thin at this layer of the atmosphere. The top of the exosphere lies
at about 10000 km after which it fnally fades into space. This layer is also
completely free of water vapor.
25
Chapter 2. Terahertz Wave Propagation in ISL 2.3. Scattering Loss
Figure 2.4: Layers of atmosphere [11]
As mentioned above, layers thermosphere and exosphere of the atmosphere
does not contain any amount of water vapor. Figure 2.3 shows the impact of
water vapor on THz frequencies. Since LEO & MEO satellites are present at the
thermosphere and exosphere layer of the atmosphere while GEO satellites are
present in deep space, we can say that in THz based ISL the absorption of the
THz frequencies will be negligible. Thus, absorption will not have a considerable
afect on THz based ISL.
2.3 Scattering Loss
Scattering is a process in which some forms of radiation, such as light, change its
path from straight trajectory into one or more paths. Scattering occurs due to the
deformities present in the material medium through which the EM wave pass.
It is important to understand that refection, refraction and scattering all leads
26
Chapter 2. Terahertz Wave Propagation in ISL 2.3. Scattering Loss
to the re-direction of light. When the medium is uniform and homogeneous over
lengths that are much larger than the wavelength then a well-defned direction of
propagation emerges. This is refraction. However, when the medium is smaller or
comparable to the wavelength then the wave goes of in diferent direction. This
is scattering.We can say that refraction is a form a scattering. Scattering of the
electromagnetic waves corresponds to the collision and scattering of a wave with
some material object.
Types of Scattering
Depending upon the size of the material object with which the EM wave colloids,
scattering can be categorized into three.
∙ Rayleigh Scattering – This occurs when the size of particle is small in
comparison to the wavelength of the wave.
∙ Mie Scattering – This type of scattering occurs when particle is about the
same size as wavelength of the wave
∙ Geometric Scattering – This is caused when the particle is much larger than
the wavelength of wave
In order to do the analysis of the impact of scattering on THz Based Inter-satellite
communication, we should frst analyze the type of scattering which occurs at the
layers where satellites are present.
In case of optical/laser communication geometric scattering is mainly caused by
rain droplets and snow [15]. Aerosol particles,fog and haze are major contributors of
mie scattering [14] while rayleigh scattering is mainly caused by air molecules [16].
As we discussed above, in thermosphere and exosphere the only thing which is
present is air molecules with very low density. Now, in our scenario we are trans-
mitting at THz frequency which means at higher wavelength than the wavelength
27
Chapter 2. Terahertz Wave Propagation in ISL 2.4. Fading Loss
used in optical communication. So, we can infer that only scattering which is
possible in THz based ISL is rayleigh scattering.
In the case of rayleigh scattering the relation between amount of scattering
and wavelength is given by [13]
1 � ∝ (2.7)
�4 ,
where � is the wavelength of the THz wave and I is the intensity of the light
scattered
As described in [12], rayleigh scattering is neglected at wavelengths in the near
infrared range (0.75 �m-1.4 �m). In THz based ISL,we are transmitting at
wavelengths higher than the wavelength in the near infrared range. Due to the
inverse relationship shown in equation (2.7), the impact of scattering in THz
based ISL is even lesser in comparison to the scattering in near infrared range.
Furthermore, the density of the air molecules in the upper layers of the atmosphere
is very less.Thus,we can conclude that when we are using terahertz band for inter
satellite communication there cannot be huge impact on the signal quality due to
scattering.
2.4 Fading Loss
There are mainly three mechanisms through which an EM wave propagate in a
terrestrial wireless medium; refection, which is a type of geometric scattering,
difraction and rayleigh scattering.These mechanisms lead to arrival of multiple
electromagnetic waves at the receiver at the same time. These waves usually arrive
at receiver from diferent directions and experience diferent delay due to traversing
unequal distances. The received signal amplitude and phase of each of the wave
can difer as they travel diferent distances. As a result, these EM waves can add
28
Chapter 2. Terahertz Wave Propagation in ISL 2.4. Fading Loss
either constructively or destructively. Due to this, we can observe fuctuations
in the received signal power. Furthermore, the received signal can vary rapidly
in phase and amplitude if there is relative motion between the transmitter and
receiver.
When an EM wave propagates from transmitter to receiver, signal sufers one
or more refections.This leads to multi path propagation. Due to unequal distances,
time of arrival of each path at the receiver is diferent. This leads to spreading out
of the signal in time. This spread is called as delay spread. As we will discuss in
the future chapter, high delay spread can lead to inter symbol interference.
Fading also occurs when there is relative motion between transmitter and re-
ceiver. Due to doppler efect, relative motion leads to generation of new frequencies
at the receiver. In chapter 5, we will see that high speed of the transmitter or
receiver can lead to very rapid change in the channel response causing distortion
in the received signal.
29
CHAPTER 3
Noise in Terahertz based ISL
Noise is an unwanted disturbance that gets added during the capturing or processing
of the desired signal. In this chapter we will mainly look at the type of noise in
free space and analyze their impact on THz based inter satellite communication.
3.1 Thermal Noise
Thermal noise is the electronic noise generated by the motion of the charged
particles such as electrons inside an electrical conductor. Except at absolute zero
temperature, the electrons in every conductor (resistor) are always in thermal
motion. This noise is generated even when there is no voltage applied to the
conductor. This motion leads to voltage diference between the resistor’s terminal.
As shown in Figure 3.1, this can be modeled as resistor with the noise source.
30
R (rhy~-.s.1) kTB - (id&I)
Chapter 3. Noise in Terahertz based ISL 3.2. Transceiver Noise Sources
Figure 3.1: Non ideal resistor & its equivalent scheme [1]
As mentioned in [2], if this resistor is connected to a matched load the thermal
noise delivered by the noise source will be equal to
�� = �� �, (3.1)
where K is the Boltzmann constant in Joules per Kelvin (� = 1.38 × 10−23 J/ K),
T is temperature in kelvin and B denotes bandwidth in Hz.
We can use equation (3.1) to defne an efective noise temperature for the other
noise sources too, even if the origin of the noise is not thermal. Any noise source
having a noise temperature of �� will generate thermal noise equal to a resistor
at temperature ��. It is very important to note that, unlike resistors noise
temperature of any noise source is not always equal to its physical temperature.
3.2 Transceiver Noise Sources
In this section we will defne noise fgure and see the efective noise temperature of
some of circuit components important to our analysis.
3.2.1 Amplifer Noise Temperature
As discussed in Sec.3.1, we can defne efective noise temperature for any type
of noise source. Consider a non ideal low noise amplifer (LNA) with gain G. As
31
Chapter 3. Noise in Terahertz based ISL 3.2. Transceiver Noise Sources
shown in Figure 3.2, this amplifer has input signal ��� with some noise ���. Apart
from ���, this amplifer will add some extra noise to the signal. As described in
Sec.3.1, in a similar way this amplifer can be replaced by an ideal amplifer and
an artifcial noise source ���.
Figure 3.2: Non ideal LNA & its equivalent scheme [1]
Noise Factor
The noise factor is defned as the signal-to-noise ratio between input and output of
the system. The noise factor of the LNA described in Figure 3.2 can be given by
����� = 1 + ���
� = (3.2)������ ���
In order to make it possible to compare diferent amplifers, noise fgure is calculated
by keeping a reference temperature of 290 K. This artifcial noise can now be
thought of as fxed. At a temperature of 290K,the value of the noise power per
unit hertz can be given by
�0 = ��0 = 1.38 × 10−23 × 290 = 4 × 10−23�/��. (3.3)
The noise fgure is simply F expressed in decibel [1]
������ ����� = 10���10�. (3.4)
32
Ampliriief l I Ampiifie r 2 -G, Na. ;i G2 T~1 T,12 .__
!
No.1
Chapter 3. Noise in Terahertz based ISL 3.2. Transceiver Noise Sources
Noise Temperature
As we can see from equation (3.3) the value of noise power is very low. Sometimes
it becomes to difcult to deal with these small values. We can rewrite equation
(3.2) to
= 1 + ���
� , (3.5)�0
which can be be further rewritten to
��� = (� − 1)�0 = (� − 1) × 290, (3.6)
where ��� denotes the efective temperature noise of the LNA, �0 is the reference
temperature (290K) and F is the noise fgure.
3.2.2 Amplifers in Cascade
It is possible to calculate the efective noise temperature of the amplifer connected
in cascade. The cascade connection is shown in Figure 3.3 The overall gain is
� = �1�2. (3.7)
Figure 3.3: LNA in cascade [2]
33
Chapter 3. Noise in Terahertz based ISL 3.2. Transceiver Noise Sources
System noise temperature in this case can be given by [2]
��2�� = ��1 + , (3.8)
�1
where �� is the system noise temperature, ��1 is the noise temperature of the frst
amplifer, �1 is the gain of the frst amplifer and ��2 is the noise temperature of
the second amplifer. Using equation (3.8), the total system noise can be given by
�0,1 = ���. (3.9)
This is a very important result. It shows that in order to have low system noise
temperature, the frst stage should have high power gain as well as low noise
temperature. This result may be generalized to any number of stages in cascade [1].
The system noise temperature of more than two amplifers in cascade can be given
by ��2 ��3
�� = ��1 + + + ... (3.10)�1 �1�2
3.2.3 Noise Temperature of Absorptive Networks
A network which contains resistive elements is considered to be an absorptive
network. Since an absorptive network contains resistance it generates thermal
noise. Transmission lines, feeder lines and waveguides are some of the examples of
the absorptive networks. Feeder line is generally used to connect to the satellite
antenna. The efective noise temperature of an absorptive network is directly
related to the power loss L.
Consider an absorptive network which has a power loss L. The power loss is
the ratio of the input signal power to the output signal power. It is always greater
than unity. From [1], the efective noise temperature of an absorptive network is
34
Chapter 3. Noise in Terahertz based ISL 3.3. Antenna Temperature
given by
��� = (� − 1)��, (3.11)
where �� is the physical temperature. On comparing with equation (3.6), we can
see that if an absorptive network is at room temperature(290K) then its power
loss is equal to its noise factor.
3.3 Antenna Temperature
Antennas operating in the receiving mode introduce noise in the satellite circuit.
The temperature associated with the noise at the terminals of the antenna is known
as antenna temperature. We can think of it as the efective noise temperature of the
antenna. The noise energy due to the antenna temperature is the weighted average
of the noise sources the antenna is looking at. The weighting factor depends upon
the directivity towards a specifc noise source. Antenna temperature is due to
the presence of radiations in the antenna environment and antenna losses. It is
important to note that the antenna temperature is not the physical temperature
of the antenna. The antenna temperature is usually calculated experimentally.
But, it is possible to fnd an upper bound on the antenna temperature depending
upon the scenario.
Under Rayleigh-Jeans approximation (h� << KT), the antenna temperature
is given by ∫∫ 1 �� = �(�, �)�� �Ω, (3.12)4� Ω
where �(�, �) is the radiation pattern of the antenna and �� is the brightness
temperature.
35
Chapter 3. Noise in Terahertz based ISL 3.3. Antenna Temperature
3.3.1 Brightness Temperature
Brightness temperature is the physical temperature that black body in thermal
equilibrium would have to be in order to radiate the same power as of the non-black
body at a frequency �. Brightness temperature of a body varies with the frequency.
At any temperature and at any frequency, the power radiated by a black body is
more than a non-black body. As a result, the brightness temperature of a noise
source is always less than its physical temperature.
In the case of large bright noise object, the antenna temperature due to the
noise source is equal to its brightness temperature. Figure 3.4 depicts a large
bright noise source with brightness temperature �� . The antenna temperature for
a large bright noise source is independent of the radiation pattern and the distance
between the antenna and noise source.
Figure 3.4: Large bright noise source with brightness temperature �� [2]
However, antenna temperature does depend upon the distance and the radiation
pattern in the case of small bright objects. Figure 3.5 shows a small bright object.
It is very evident from Figure 3.5 that if the distance between the noise source
and antenna changes the solid angle (Ω� ) subtended by the noise source at the
antenna will change. The antenna temperature due to a small bright noise source
36
@·---------~-{-~ ---~,,_( ~~~--:~~~~~~~~:-----------------c~~~.-~ .... --:===--•--
Chapter 3. Noise in Terahertz based ISL 3.3. Antenna Temperature
is given by Ω�
�� = �� , (3.13)�
where Ω� is the solid angle of the main lobe of antenna.
Figure 3.5: Small bright noise source [2]
Now we will look at some of the possible noise sources which could contribute
towards antenna temperature in THz based ISL.
3.3.2 Cosmic Microwave Background Radiation
Cosmic Microwave Background Radiation (CMBR) flls the entire universe and
is the leftover radiation from the big bang. According to Big Bang, initially the
universe was flled with hot plasma of particles (protons, neutrons and electrons)
and photons (light). For the frst 380,000 years the photons were interacting
with electrons and were not able to travel long distances. However, the universe
expanded and the temperature came down to 3000 kelvin. Now the protons and
electrons were able to combine to form hydrogen atoms. In the absence of free
electrons, the photons were able to travel more distances and thus the universe
became transparent.Over the billions of years, universe has expanded and cooled
greatly and the temperature has reduced to 2.7 K
In 1989, NASA sent a Cosmic Background Explorer (COBE) to study about
CMBR. The key discovery was that CMBR was found to be extremely precisely to
a so called black body at a temperature of 2.73 K. Since the peak of CMBR lies
37
Chapter 3. Noise in Terahertz based ISL 3.3. Antenna Temperature
in the microwave region, the brightness temperature at THz frequencies due to
CMBR is very less. Needless to say, the brightness temperature of CMBR cannot
go above 2.73 K. As a result, the impact of CMBR on THz based ISL is negligible.
3.3.3 Thermal Radiation
Any body above a temperature of 0K, emits out electromagnetic radiation. These
radiation are known as thermal radiation. These radiations are generated due to
the motion of the charged particles in the matter. Some of the examples of thermal
radiation are infrared light and visible light. The peak of the thermal radiation
depends upon the temperature of the object.The peak lies in infrared region at
room temperature (290 K). Thermal radiation from the Earth has considerable
impact on satellite antennas. This is because satellite antennas generally point
towards the earth. As a result, they receive full thermal radiation from it. Since
the size of satellite antenna is very small in comparison to the size of the noise
source, i.e., Earth, the antenna temperature due to the thermal radiation from
Earth is considered to be 290 K which is the physical temperature of the Earth.
3.3.4 Sky Noise
A body which absorbs radiation also emits out radiation. The amount of the
radiation a body emits depends upon the absorptivity of the body. A perfect
black body has emissivity and absorptivity both equal to 1. Consider, e.g., there
is no atmosphere and the antenna at the earth station is pointing towards the
space. In this case the only noise source the antenna is looking at is the cosmic
noise, and the antenna temperature would be 2.7 K. But since the atmosphere
absorbs radiation, it also emits some radiation of its own. These radiation acts
as noise at the antenna and contribute signifcantly to the antenna temperature.
This contribution is known as sky noise. Sky noise depends upon the frequency
38
Galac1ic•r..oise region Lo w.no ise region
10,000 ;::::::::::'.:::::: ::::;.::::::: :::'.:::::::::::;::::::::: :'.:::::::::::::
10
Frequeocv. GHt
resonance 22.2 GHz
100
Chapter 3. Noise in Terahertz based ISL 3.3. Antenna Temperature
and the direction at which the antenna is pointing.
Figure 3.6 depicts the antenna temperature due to the sky noise for an earth
station antenna. The lower graph is for the antenna pointing directly overhead
while the upper graph is for the antenna pointing just above the horizon. The
diference between the two curves is due to the thermal radiation from the Earth
at 290 K. When the antenna is pointing to zenith, then antenna temperature is
only due to the sky noise. However, when the antenna points towards the horizon
then due to the thermal radiation from the earth the antenna noise temperature
increases.
Figure 3.6: Antenna temperature of an ideal ground based antenna. The antenna is assumed to have very low beamwidth without losses and sidelobes [2]
Notice the two peaks in the antenna noise temperature above 10 GHz. These
peaks are due to resonant losses in the Earth’s atmosphere by �2�����2. In the
case of rainfall,the atmospheric absorption is even more and hence the antenna
noise temperature increases.We can say that rainfall degrades the transmission in
two ways, it attenuates the signal as well as it introduces noise in the system.
39
Chapter 3. Noise in Terahertz based ISL 3.4. Total System Noise
3.4 Total System Noise
In this section we will calculate the overall system noise temperature.The con-
stituent elements of the receiving satellite are divide into antenna element, the
feeding section (outside the satellite), the feeding section (inside the satellite) and
the receiver (inside the satellite).Using equations (3.6, 3.10, 3.11) and as mentioned
in [26], the total system noise can be given by
���� = �� + (�1 − 1)�1 + �1(�2 − 1)�2 + �1�2(�� − 1)�0, (3.14)
where �� is the antenna temperature, �1, �2, �0 are the physical temperature of the
feeder line (outside the satellite), feeder line (inside the satellite) and the receiver
respectively. �1&�2 are the loss in the feeding section (outside the satellite) and
feeding section (inside the satellite) respectively, and NF is the noise fgure of
the receiver. We will be using this equation to calculate the total noise power
generated in THz based inter-satellite communication.
Now the total noise power �� at the receiver can be readily found as
�� = ������, (3.15)
where K is the boltzman constant and B is the bandwidth.
It is important to note that the temperature outside the satellite depends
upon the location of the satellite in the orbit. In order to fnd the range of the
temperature outside the satellite, we frst need to understand how a body attains
temperature. There are three ways through which heat is transferred from one
body to another.
∙ Conduction - In conduction there is a direct fow of heat through a body as
a result of physical contact from another body. The former body can either
gain or loose heat which can eventually lead to the increase or decrease in
40
Chapter 3. Noise in Terahertz based ISL 3.5. Signal-to-Noise ratio
the temperature of the body.
∙ Convection - In case of convection, heat is transferred to the body by the
fuids such as gas or liquid. Heat transferred to the body by the collisions of
the atmospheric molecules is an example of convection.
∙ Radiation - In radiation mechanism heat is transferred to a body using elec-
tromagnetic wave. It does not require any material medium for transferring
the heat.
In THz based ISL, we will be communicating in an environment where the density
of the air is very low. There will be very less air molecules to transfer the heat
via convection. So even though the temperature of thermosphere is 2000∘C (2273
K), the temperature outside the satellite is very low. Also, the heat transfer will
not take place via conduction as it requires a separate physical body for transfer.
Hence, in our analysis the only way through which heat transfer can take place is
via radiation. As a result, the temperature of LEO satellites vary from -170∘C
(103 K) to 123∘C (396 K) depending upon the location in the orbit. We have to
consider this wide variation in the temperature outside satellite in our link budget
design as it can impact the total noise power at the receiver.
3.5 Signal-to-Noise ratio
The signal-to-noise ratio (SNR) is an important measure to evaluate the perfor-
mance of the satellite link. It is the ratio of carrier power to noise power at the
receiver input. It is given by � �� = , (3.16)� ��
41
Chapter 3. Noise in Terahertz based ISL 3.5. Signal-to-Noise ratio
where �� is the received power and �� is the noise power. This equation can also
be expressed in decibels (dB)
( ) � = ���� − ���� . (3.17)� ��
� Using equations (2.6, 3.14, 3.17), we can rewrite the ratio as�
( ) � = ���� + ���� + ���� − ��� − ��� − ������ − ��� . (3.18)� ��
42
CHAPTER 4
Link Budget Analysis
From the information we have gained from the previous chapters, in this chapter
we will be devising link budget for THz based inter-satellite communication. We
will begin by calculating the gain of the transmitter and receiver antenna required
to compensate for the high propagation loss occurring due to the transmission
at THz frequencies. We will also calculate the beam width corresponding to the
gain, and then we will perform a comparative analysis on the basis of the pointing
requirements between Optical and THz based ISL.
For link budget analysis, we will make use of the Friis Propagation model
developed in Chapter 2. For calculating the noise power we will be making use of
the total system noise temperature equation developed in Chapter 3.
The recent advancements in the terahertz technology, such as III-V semicon-
ductor technologies in an electronic approach and Quantum Cascade Lasers in an
optics approach, allows us to generate frequencies from 0.34 THz to 1 THz with a
power of 1 mW to 10 mW [4, 18].
We will divide the analysis into two parts depending upon the transmit power,
carrier frequency and location of the satellite in the orbit. We desire to achieve an
43
Parameters Value
pt lOmW
{c 0.34THz
TA 300K
L1,L2 1.3 dB
Ti 103K
Tz 290K
NF 3 dB
d so km - 1000 km
B 1 GHz, 5 GHz, 10 GHz
Chapter 4. Link Budget Analysis 4.1. Antenna Gain Required - Lower Bound
SNR of 10 dB at the receiver. At frst, we will fnd the lower bound on the gain
of the antenna required to maintain an SNR of 10 dB at the receiver. We will
do the analysis by choosing three diferent values of the bandwidth. In the same
way, we will be fnding an upper bound on the gain of the antenna with respect to
distance.
4.1 Antenna Gain Required - Lower Bound
For lower bound analysis the temperature outside the satellite is taken to be
-170∘C (103 K). This is because as we decrease the temperature the noise power
decreases and the signal to noise power ratio increases. Hence, we would require
less amount of gain of the antenna to achieve the same SNR.
Table 4.1 indicates the parameters considered in the lower bound analysis.
Table 4.1: Parameters - Lower bound
44
6B Fe= 0.34 THz, Pt= 10 mW, Tsys = 683.1 K
--8=1 GHz 66 ~ B=5GHz ------ - B= tO GHz 64
62 ,,,,, ,,,,,
co / /
~ 60 /
~ /
·~ 5B /
~ I C 56
I ·;;; I 0 I
54 I
52
50
4B 0 100 200 300 400 500 600 700 BOO 900 1000
Distance[km]
Chapter 4. Link Budget Analysis 4.1. Antenna Gain Required - Lower Bound
Gain Required
Figure 4.1: Gain required for SNR of 10 dB vs Distance (Lower Bound)
From Figure 4.1, we can see that as the distance increases the gain required to
achieve SNR of 10 dB also increases. This is due to the increase in the propagation
loss with the distance. The total system noise temperature is found to be 683.1
K. We can also see that the gain required is the highest in the case when the
bandwidth is 10 GHz. This is because the total noise power is directly proportional
to the bandwidth. As a result, we will need more gain of the antenna to achieve
SNR of 10 dB.
Beamwidth
The beamwidth, also known as half power beamwidth (HPBW), is the angle
between the half power points (-3 dB) of the main lobe. In this section, we
will calculate the upper bound on the HPBW corresponding to the gain of the
transmitter and receiver antenna calculated in previous section. From [20], the
45
Fe= 0.34 THz, Pt= 10 mW, Tsys = 683.1 K 0.8 ~-~-~--~-~--~-~-~--~-~-~
0.7
O.G
ru ~ 0.5 <1> ~ ~ 04
-~ ~ 0.3 co
0.2
0.1 --- - - -
--B=l GHz ~ B=SGHz - - B=lOGHz
---------o~-~-~--~-~--~-~-~--~-~-~
0 100 200 300 400 500 600 700 800 900 1000
Distance[km]
Chapter 4. Link Budget Analysis 4.1. Antenna Gain Required - Lower Bound
gain of the antenna is related to the half power bam width as
32400 � ≤ , (4.1)
��
where � is horizontal beamwidth in degrees and � is the vertical beamwidth in
degrees. Horizontal and vertical beamwidth are the angles in the horizontal and
vertical plane where the power of the main lobe is reduced by half. We will assume
the horizontal and vertical beam width to be equal in our analysis. The relation
between HPBW and gain of the antenna can now readily found to be
√ 32400
�� = , (4.2)�
where �� is the HPBW in degrees.
Figure 4.2: Beamwidth vs Distance (Lower Bound)
From Figure 4.2 we can see that as the distance between transmitter and
receiver increases the upper bound on the HPBW decreases. This is due to the
46
Fe= 0.34 THz, Pt= 10 mW, Tsys = 683.1 K 3.5 ~-~-~--~-~--~-~-~--~-~-~
3
2.5
I ~ 2 E .!!! ~ 1.5
"' Q)
co
0.5
--8=1 GHz ~ 8=5GHz - - 8=10 GHz
o'--~--~-~--~-~--~-~--~--'---' 0 100 200 300 400 500 600 700 800 900 1000
Distance[km]
Chapter 4. Link Budget Analysis 4.1. Antenna Gain Required - Lower Bound
inverse relationship between the gain and the HPBW. It is important to note that
as the HPBW decreases it becomes difcult to aim at the receiver. Hence, there is
a need for an accurate pointing system for communicating with the receiver.
Beam Diameter
Using geometry, the beam diameter at a particular distance d of an antenna beam
with HPBW �� can be readily found as
( )
������������ = 2���� ��
. (4.3)2
Figure 4.3: Beam diameter vs Distance (Lower Bound)
From Figure 4.3, we can see that the beam diameter is increasing with distance.
It is very important to note that even though the HPBW is decreasing the beam
diameter is increasing and it is of the order of kilometer (km).
47
Parameters Value
pt lmW
fc l THz
TA 300K
L1,Lz 1.3 dB
T1 396K
Tz 290K
NF 3dB
d so km - 1000 km
B 1 GHz, 5 GHz, 10 GHz
78 Fe = 1 THz, Pt= 1 mW, Tsys = 712.4 K
--B=1 GHz 76 -e-e=SGHz ------ - B=lOGHz --74
72 ,,,. iii'
,,,,
~ 70 /
/
~ / -~ 68 / ~ I
~ 66 I
~ I
64 I
I
62
60
58 0 100 200 300 400 500 600 700 800 900 1000
Distance[km]
Chapter 4. Link Budget Analysis 4.2. Antenna Gain Required - Upper Bound
4.2 Antenna Gain Required - Upper Bound
In the same way we will repeat the calculations when the temperature outside the
satellite is 123∘C (396 K). In upper bound analysis we will consider the transmit
power to be 1 mW and the carrier frequency to be 1 THz.
Table 4.2 indicates the parameters considered in the lower bound analysis.
Table 4.2: Parameters - Upper bound
Figure 4.4: Gain required for SNR of 10 dB vs Distance (Upper Bound)
48
Fe = 1 THz, Pt= 1 mW, Tsys = 712.4 K 0.3 ~------------------------~
0.25
QJ 0.2 ~ i ~ ~ 0.15
-~ ro Q)
Ill 0.1
0.05 ------
--8=1 GHz ~ 8=5GHz - - 8=10GHz
-----o~-~--~-~--~-~--~-~--~--~-~
0 100 200 300 400 500 600 700 800 900 1000
Distance[km]
Fe = 1 THz, Pt= 1 mW, Tsys = 712.4 K
--8=1 GHz 0.9 ~ 8=5GHz
0.8
"f 0.7 6 .; ., 0.6 E ro ~ 05 ro Q)
Ill 0.4
0.3
0.2
- - 8=10 GHz
0.1 ~-~-~--~-~--~-~-~--~-~---' 0 100 200 300 400 500 600 700 800 900 1000
Distance[km]
Chapter 4. Link Budget Analysis 4.2. Antenna Gain Required - Upper Bound
Figure 4.5: Beamwidth vs Distance (Upper Bound)
Figure 4.6: Beam diameter vs Distance (Upper Bound)
The efective noise temperature in the upper bound analysis is 712.4 K. On
49
Chapter 4. Link Budget Analysis 4.3. Terahertz ISL vs Optical ISL
comparing the corresponding fgures in the lower bound and upper bound analysis,
we can say that there is a diference of about 10 dB in the gain of the antenna
required to achieve an SNR of 10 dB. It is also important to note that the
beamwidth is as low as 0.05 degrees in the upper bound analysis. This will lead to
the increase in the pointing requirements between transmitter and receiver.
4.3 Terahertz ISL vs Optical ISL
In this section, we will do a comparative analysis between the THz based ISL and
optical/laser based ISL. This comparison will be done on the basis of pointing
requirements in both the cases.
From [21], the beam diameter of a gaussian laser beam at a position z is given
by √ ( )2�
�(�) = 2�0 1 + , (4.4)��
where ��0
2
�� = , (4.5)�
and z is the axial distance, and �0 is the waist size. Waist size is the radius of the
laser beam at its narrowest point.
In our analysis, we will consider a red laser with wavelength of 650 nm.We
will assume the waist size to be 1 cm.This value is deliberately taken to be large
in order to show the big diference in the beam diameter of THz based ISL and
optical/laser based ISL. We will calculate the beam diameter for the same range
of distances, i.e., 50 km to 1000 km.
50
0.045
E o.04 6 .; a3 .~ 0.035
~ ~
CO 0.03
0.025
0.02 L--=--~--~-~--~-~-~--~-~---' 0 100 200 300 400 500 600 700 800 900 1000
Distance[km]
Chapter 4. Link Budget Analysis 4.3. Terahertz ISL vs Optical ISL
Figure 4.7: Beam diameter in Optical ISL
From Figure 4.7, we can see that the beam diameter is very less in the case
of optical/laser based ISL. Also, from Figure 4.8 the beam diameter in lower
bound analysis and upper bound analysis at 10 GHz is much higher than the
beam diameter in optical/laser based ISL. As a result, pointing requirements in
the case of optical/laser based ISL will be very high in comparison to THz based
ISL. We will need a highly accurate ATP subsystem in order to communicate with
the receiver. Thus, we can say that the efciency of THz based ISL is more in
comparison to optical based ISL.
51
0.5 ~------------------------~
0
I ;;; a3 -0.5 E .!!!
~ ro .., -1 ~ en ..9
-1 .5
--THz l ov.oer bound - - THz Upper bound -G- Laser
/' /
,,,,. ,,,,. --
--- ------- -------
-2 ~-~--~-~--~-~--~-~--~--~-~ 0 100 200 300 400 500 600 700 800 900 1000
Distance[km]
Chapter 4. Link Budget Analysis 4.3. Terahertz ISL vs Optical ISL
Figure 4.8: Optical ISL vs THz ISL
In this chapter, we devised link budget analysis and calculated the gain of the
transmitter and receiver antenna required to achieve an SNR of 10 dB. In the
worst case scenario of maximum distance between satellites (1000 km), maximum
noise temperature of 712.4 K, transmit power of 1 mW, carrier frequency of 1 THz
and bandwidth of 10 GHz, we have found the gain required to be 76 dB. This
high gain of the antenna can be achieved by using an array or refector antenna.
A mechanically scanned confocal ellipsoidal refector antenna system was reported
in [22]. The simulation results show to achieve a gain of 75 dB at 0.67 THz.
52
CHAPTER 5
Additional Challenges in THz based ISL
In this chapter we will look at some of the additional challenges that needs to be
addressed in THz based ISL. We will begin by looking at the possibility of multi
path propagation due to the presence of debris in the space and its impact on the
quality of communication. We will also consider the impact of the high relative
motion between the satellites.
5.1 Space Debris
Space debris is the collection of non-functional human made objects in the earth
orbit such as old satellites, fragments from disintegration, erosion and collisions.
As of July 2013, more than 170 million debris smaller than 1 cm, about 670,000
debris 1-10 cm, and around 29000 larger debris were estimated to be in orbit.
Most of these debris are present in LEO and MEO [23]. In our analysis,we are
making use of THz radiation which means a wavelength of around 10−4 m. From
Sec.2.4, we can say that the scattering which will occur due to the space debris
will be geometric scattering. We need to take into account the refections which
53
Chapter 5. Additional Challenges in THz based ISL 5.1. Space Debris
will occur due to the space debris which can lead to multi path propagation.
In the next section, we will develop a simulation model for the multi path
propagation due to space debris in THz based ISL.
5.1.1 System Model
To model the number of multipath components, we need to take into account
propagation loss, the geometry of the network and the density of space debris.
LOS & NLOS propagation loss
Using equation (2.5), we can say that the total path loss for the line-of-sight (LOS)
component can be given by
( )24��� � ���� (�, �) = , (5.1)
�
where f is the operating frequency in Hz, d is the separation between the transmitter
and receiver, c is the speed of light in free space.
For the components which are not in line-of-sight (NLOS),we need to account
for the refection loss which occurs when the signal collides with an obstacle. In
this case, the total path loss can be given by
( )24��� � ����� (�, �) = �(�), (5.2)
�
where �(� ) is the refection coefcient.
Network Topology
The origin of the coordinate system is assumed to be at the mid-point of the distance
between two satellites. The transmitting and receiving satellites are placed at
equal distance d/2 from the origin on the x axis. We consider directional antennas
54
Chapter 5. Additional Challenges in THz based ISL 5.1. Space Debris
�2
�
� � ��
(−�/2, 0) �1
(�/2, 0)
Figure 5.1: Geometry of the Network considered. Refection occuring at point A.
are deployed at both the transmitter and receiver with �� as the beamwidth. We
also consider that diferent obstacles are randomly distributed in the space by
following a spatial Poisson process with rate �. Since poisson process is a stationary
independent increment process, the probability of fnding k objects in the area A
is given by (��)�
�−�� � (�∈�) = . (5.3)�!
As shown in Figure 5.1, due to an obstacle (space junk) at point A there is
refection of the signal. As a result, another copy of the same transmitted signal is
received at ��. In our analysis, we have considered only single bounce refections.
5.1.2 Simulation Model
In this section we will perform simulations to fnd out the number of multipaths
due to space debris in THz based ISL with respect to time. We chose the frequency
55
1 x 104
0.8
0.6 + + 0.4 +
E 0.2 ij. +
.r u C 0 + ++ ro cii + + 0 -02 + + +
-0.4 + + + -0.6
-0.8
-1 -5 -4 -3 -2 -1 0 1 2 3 4 5
Distance[m] x 105
Chapter 5. Additional Challenges in THz based ISL 5.1. Space Debris
at which we are transmitting to be 0.34 THz. The value of of beamwidth is
considered to be 0.5 degree. The distance between the satellites is considered to be
1000 km. Also, the value of refection coefcient is considered to be 1. The fnal
result was found by taking 1000 realizations of the system. The simulation result
is calculated by plotting the histogram of the number of multipaths with time.
In our analysis we have considered the density of the space debris to be 10-9 per
�2. We can see from the Figure 5.2 all the space debris that are enclosed in the
receive and transmit beam. These space debris will lead to multipath propagation.
Figure 5.2: Space debris enclosed in Transmit & Receive Beam,� = 10-9 per �2
56
4
3.5
,,, 3 .c ro _g. 2.5 "3 E 0 2 0 z
1.5
0.5
o~-~-~-~--~-~-~-~~-~-~-~ 3.3333 3.33332 3.33334 3.33336 3.33338 3.3334
Delay[ms]
Chapter 5. Additional Challenges in THz based ISL 5.1. Space Debris
Figure 5.3: Multipath components as function of path delay, � = 10-9 per �2
From Figure 5.3, we can see that as time increases the number of multipath
decreases. We can observe that the frst path arrives at the receiver at time 3.33
ms. This is because we have considered distance between transmitter and receiver
to be 1000 kilometer and the speed of light is 3 × 108 m/s. This plot is drawn
by calculating the total distance traversed by the signal from the transmitter to
the receiver via the refector. This total distance is divided by the speed of light
to calculate the time at which signal will arrive at the receiver. After that a
histogram is plotted for all the multipaths. This experiment is conducted 1000
times to calculate the average number of multipaths with time.
The model that we have developed in this chapter is very general which means
that it is independent of the technology used in PHY layer. This model can also
be extended to terrestrial environment with directional antennas. However, the
model that we have described above will change in the case of a bounded area
application.
57
Chapter 5. Additional Challenges in THz based ISL 5.1. Space Debris
From Figure 5.3, we can also see that the delay spread is about 0.1 ��. As
mentioned in Chapter 2, a high delay spread can lead to Intersymbol Interference
(ISI). ISI happens when the duration of the signal that we are transmitting is less
than the delay spread. In our scenario, since the bandwidth is in the order of GHz,
the duration of the signal will be of the order of nanoseconds. As a result, there
are chances of ISI in THz based ISL. So we need to take into account the impact
of space debris on the quality of the link.
5.1.3 Power Delay profle
In this section, we will develop a power delay profle for THz based ISL in presence
of space debris. In order to fnd the power delay profle we need to know the
probability that any multipath component does not get blocked in the way while
reaching at the receiver. We will consider the obstacles to be of circular shape with
radius r and they follow the same Poisson distribution as mentioned in Sec.5.1.1.
As described in [18], the probability that the signal does not get blocked in the
way while reaching at the receiver is given by
2)��� (�) = �−�(��+�� , (5.4)
where r is the radius of the object and d is the distance traveled by the multipath
component from the transmitter to the receiver. The power delay profle can now
be readily found by � ����� (�, �� )
� �� (� ) = (5.5)�(�)��� (� = �� )
.
In our analysis,we have assumed the radius of the obstacles to be 5 cm and the
carrier frequency is taken to be 0.34 THz.
58
-198
cii'-200 ~ Q)
0: e -202 0. >,
"' -.; -o -204 t,:;
~ a. -206
-208
-210 ~-~-~--~-~--~-~-~--~-~-~ 3.3333 3.33332 3.33334 3.33336 3.33338 3.3334
Delay[ms]
Chapter 5. Additional Challenges in THz based ISL 5.2. Doppler Spread
Figure 5.4: Power delay profle vs time
From Figure 5.4 we can say that the power received at the receiver is decreasing
with time. This is because the multipath components which are arriving late at
the receiver are traveling more distance. Furthermore, the number of multipaths
are also decreasing with time.
5.2 Doppler Spread
Another major challenge that we need to take care in THz based ISL is the issue
of doppler spread due to the high relative motion between the satellites. Doppler
efect occurs when the receiver or the transmitter is moving with respect to the
wave source. In this scenario the frequency received at the receiver is not the same
as the source. If the transmitter and receiver are moving towards each other then
the frequency of the EM wave at the receiver increases. On the other hand, if the
transmitter and receiver are moving away from each other then the frequency of
the EM wave at the receiver decreases. The frequency of the received signal ��
59
Chapter 5. Additional Challenges in THz based ISL 5.2. Doppler Spread
can be given by
�� = �� − ��, (5.6)
where �� is the transmitter frequency and �� is the doppler frequency [24]. The
doppler frequency is given by �
�� = , (5.7)�
where v is the relative speed in the moving direction and � is the wavelength of
the EM wave.
In our analysis, the impact of the doppler spread can be very signifcant.
This is because the relative speed at which the satellites move can get very high.
Furthermore,we are transmitting at very low wavelength of about 30 �� to 3000
��. This high speed of the motion between the transmitter and receiver leads
to change in the impulse response of the channel. If the time within which the
channel changes is more than the duration of the signal then channel is considered
to be slow fading channel. On the other hand, if the channel response changing
time is less than the duration of the signal then the channel changes during the
transmission of the signal and there is distortion at the receiver. This channel is
considered to be fast fading channel.
In order to fnd the maximum duration of the signal that can be transmitted
without distortion at the receiver, we defne a measure called coherence time.
Coherence time is the statistical measure of the time duration over which the
channel impulse response is considered to be not varying [24].
Coherence time is related to the doppler frequency and is approximately given
by 9
�� = , (5.8)16���
where �� is the doppler frequency.
As calculated in [25], the maximum relative speed between satellites can go
60
Chapter 5. Additional Challenges in THz based ISL 5.2. Doppler Spread
as high as 8 km/sec. In our analysis we are making use of �� = 0.34� ��. Using
equation (5.7), we can calculate the doppler frequency to be 9 MHz. We can use
this value to calculate the coherence time. The value of the coherence time is
around 0.02 ��. Thus, if we want to transmit a signal without distortion at the
receiver the duration of the symbol should be less than 0.02 ��.In our analysis,
we used the transmission bandwidth to be 1 GHz, 5 GHz and 10 GHz. Since the
duration of the signal has an inverse relationship with the bandwidth, the duration
in our case would be of the order of nanoseconds. As a result, we will not be facing
the issue of fast fading at the receiver. However, we still need to compensate for
the doppler spread at the receiver.
61
CHAPTER 6
Conclusions and Future Work
In this research, we studied the feasibility of THz band for Inter-satellite communi-
cation. We developed the propagation model and analyzed the factors which can
afect THz wave propagation in ISL. We also studied about the noise sources which
can impact the quality of the signal. In order to maintain a specifed quality of the
link, we calculated the gain of the antenna required at the transmitter and receiver
in the worst case scenario. Based upon the pointing requirements, a comparative
analysis between laser based ISL and THz based ISL was also conducted and it
was shown that the latter has more efciency. Lastly, we analyzed the impact of
space debris leading to multipath propagation and doppler spread on THz based
ISL.
As part of future work, we need to design the modulation techniques to be
used for implementing THz based ISL. Also, the value of beamwidth required
to maintain the quality of the link is low, as a result, an accurate tracking and
pointing subsystem needs to be implemented on the satellites. Furthermore, we
need to properly choose high gain antennas considering the budget on the size of
the payload.
62
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